1. Field of the Invention
The present invention relates to a bolt heater used for fastening and loosening bolts of a steam turbine casing, etc., applicable for a heating system for heating large size bolts of other various machines and for heating a narrow and long hole from the inside thereof, and usable where heating cannot be done from the outside such as a heat treatment within a narrow hole, heating of a boiler tube, heating of an inner tube of a double tube and improvement of stress in a welded section of a narrow hole.
2. Description of the Related Art
A bolt heater is used for fastening or loosening a large size bolt which is used for a steam turbine casing for example and which cannot be fastened by a wrench or spanner.
A case when a bolt heater is used for loosening a bolt will be explained referring to FIGS. 5 and 6. FIG. 5 shows a pair of flanges (C) strongly fastened by a bolt (a) and nuts (b). When a bolt heater (d) is inserted to a center hole (f) bored at the center of the bolt (a) in the longitudinal direction thereof and the bolt (a) is heated by the bolt heater (d), the bolt (a) thermally expands in the axial direction by a length (1) as shown in FIG. 6. Thereby, the nut (b) can be rotated easily and be loosened.
In this case, it is only necessary to heat a parallel portion (e) of the bolt (a), and it is preferable to avoid heating the screw portions of the bolt and nut to the extent possible.
In fastening the bolt, the bolt (a) is similarly heated up to thermally expand in the axial direction thereof. When the bolt (a) is expanded, the nut (b) is rotated in the direction of fastening the bolt (a). Then the bolt (a) is cooled down to obtain a predetermined fastening force.
Conventionally, a bolt heater constituted by a resistance wire heating element is inserted into center holes of bolts for lateral joint faces or the like of a steam turbine casing for use on land or aboard ship, in order to fasten or loosen the bolts.
Referring now to FIG. 4, one example of a prior art heater constituted by a resistance wire will be explained. In the figure, resistance wire heating elements 21 are embedded in a heat resistant insulator 22. A jacket 23 is made of a heat resistant metal such as SUS. A handle 27 is attached to a terminal box 24. Terminals 25 and 25' connect the resistance wire heating elements 21 with a cable 26. The reference numerals 28 and 28' indicate cross sections of an object to be heated such as a bolt.
In FIG. 4, when power is supplied by connecting the cable 26 to a power supply which is not shown, the resistance wire heating elements 21 generate heat and the jacket 23 is heated up through the intermediary of the heat resistant insulator 22.
However, because the resistance wire heating elements 21 are embedded in the heat resistance insulator 22 within a narrow stainless tube in the conventional bolt heater shown in FIG. 4, a quantity of heat input is limited, and because the heating is carried out indirectly, it is inefficient. Due to that, it takes a long time to fasten or loosen the bolts and has been a bottleneck in the assembly process for a turbine. Further, because it takes a long time for heating, and the resistance wire heating element 21 is exposed in a high temperature state for a long time, it is deteriorated appreciably.
In addition, because the nut portion and flanges of the casing are also heated up and thermally expanded due to the long heating, an error is brought about in a necessary elongation length of the bolt, thereby complicating its management.
Because the temperature does not rise quickly, not only are a number of heaters required (also for bolts whose length differ), but also there is a danger that an operator may touch the hot nuts and casing flanges.
Accordingly, it is an object of the present invention to eliminate the aforementioned problems by providing a high frequency bolt heater which can heat up the inside of a narrow hole much faster than the prior art bolt heater using the resistance wire heating element.